In some engines, an electronically controlled throttle is used for improved performance. In particular, the electronic throttle is used to control airflow to a desired value determined from operating conditions and an operator command. In this way, the vehicle can achieve improved drive feel and improved fuel economy.
In this system, the required airflow is used to determine an initial setting of the throttle. Also, a difference between required airflow and actual measured airflow is used to adjust the initial setting of the throttle. Thus, the throttle is used to control airflow and thereby engine torque. Such a system is described in U.S. Pat. No. 5,019,989.
The inventors herein have recognized a disadvantage with the above approach. In particular, a disadvantage with using throttle position is that the throttle cannot quickly change engine torque since the throttle controls flow entering an intake manifold. Controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. For example, if the throttle is instantly closed, cylinder air charge does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero.
Also, other methods are known for controlling engine torque where ignition timing is used. In particular, to maximize fuel economy, ignition timing should be at MBT timing (ignition timing for maximum torque). However, when at this point, adjustment of ignition timing in any direction decreases engine torque and fuel economy. Therefore, when maximizing fuel economy, engine torque can be rapidly increased. To be able to use ignition timing in both positive and negative directions, ignition timing must be set away from MBT timing. This allows rapid engine torque control, but at the cost of degraded fuel economy.